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Micrium provides template BSP filesfiles (net_bsp_ether.c/h and net_bsp_wifi.c/h), which should be copied to your project and modified depending on the combination of compiler, processor, board and device hardware used. However, Micrium might have BSP available for some specific Evaluation board which are not delivered. So before starting to write your own BSP, you can ask Micrium for a working sample code for a MCU, if a sample code is available you could just apply minor modification to be compatible with your compiler and board.